1. Field of the Invention
The present invention applies to the field of wireless communications systems using connected microprocessor-based computers and, in particular, to reducing interference from the microprocessor when a connected radio is active.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically are designed for lightweight, inexpensive user terminals, such as cellular telephones or wireless web devices. The user terminals are designed for radio reception in addition to any data processing tasks that are also provided. User terminal radio designers typically pay strict attention to frequency planning and spurious signal analysis. All clock frequencies in the system design including all significant harmonic frequencies and intermodulation frequencies that might occur are analyzed and accommodated or corrected. By careful selection of the clock frequencies, it is generally arranged that no significant harmonics fall inside certain frequency bands. Radio filters and shielding are relied upon to attenuate any remaining internally generated interference to acceptable levels.
When radio services are used for computer data transport, radio modems can be installed in or attached to general purpose computers or data appliances that are not designed with any particular radio system in mind. Laptop computers are typically not designed to accommodate any radio communications system. When a general purpose computer, such as a laptop computer is combined with a general purpose radio modem, such as a PCMCIA card, the radio designer does not have the luxury of confining all interference clock frequencies to special values which do not have harmful harmonic components. As a result, the computer can interfere significantly with the radio's reception. In extreme cases even transmission can be affected. The modulators, down-converters, amplifiers and even RF stages can all be impaired by interference generated by the computer.
The level of interference from the general purpose computer increases, as the clock speed increases. For computers with clock speeds in the gigahertz range, high frequency signals are generated inside the computer which are in commonly used data communications radio bands or in the intermediate frequency bands used by a heterodyne radio receiver. Much of the interference comes from connectors and buses between components in the computer, however, some components such as disk drives can generate significant interference notwithstanding any bus activity.
It is possible to design the general purpose computer to work well with radio components in a manner similar to that for cellular telephones and mobile data terminals. This, however, can add significantly to the design cost and any shielding will add to the manufacturing cost and weight of the computer. The designer would have greater design freedom if radio interference could be ignored. If a radio is added to a computer using, for example a PCMCIA (Personal. Computer Memory Card International Association), Compact PCI (Peripheral Component Interconnect) or SDIO (Secure Digital Input-Output) slot, the computer designer may not be able to anticipate the receive frequencies of the radio system. If the designer does not know which radio system is being used and the receive frequencies for that system, then the computer cannot be designed to avoid interference.